Method for producing a multilayer coating and the use thereof

ABSTRACT

A process for producing a multicoat system on a substrate, in which a) an electrodeposition coating film is deposited on the substrate, b) the electrodeposition coating film is predried by heating to a predrying temperature for a predetermined period, c) a coat of a surfacer is applied to the electrodeposition coating film, and d) the electrodeposition coating film and the coat of the surfacer are baked together at elevated temperatures, and in which the predrying temperature in step b) is equal to the temperature (T p ) or lies above the temperature (T p ) at which the loss factor tan δ, which is the quotient formed from the loss modulus E″ and the storage modulus E′, of the unbaked electrodeposition coating material shows a maximum, and the use of the resulting multicoat system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Application of Patent ApplicationPCT/EP01/12102 filed on 19 Oct. 2001, which claims priority to DE 100 52438.9, filed on 23 Oct. 2000.

FIELD OF THE INVENTION

The invention relates to a process for producing a multicoat system onan electrically conducting substrate, in which an electrodepositioncoating film is deposited on the substrate, the electrodepositioncoating film is predried by heating to a predrying temperature, a coatof a surfacer is applied to the electrodeposition coating film, and theelectrodeposition coating film and the coat of the surfacer are bakedtogether at elevated temperatures, and the use of the multicoat systemsobtained in this way.

The invention further relates to a method of determining the predryingtemperature of the electrodeposition coating material in a process ofthe abovementioned variety by means of dynamic mechanical thermoanalysis(DMTA). DMTA is known, for example, from the German Patent ApplicationDE 44 09 715 A1, where it is used for quantitative description of thechemical crosslinking reactions in coating films deposited on strips offabric having a defined profile of mechanical properties. By usingelectrically conductive strips of material it is also possible todeposit and investigate electrodeposition coating materials.Determination of the predrying temperature of the electrodepositioncoating films by means of DMTA is not described in DE 44 09 715 A1.

BACKGROUND OF THE INVENTION

For producing multicoat systems having a primer comprising anelectrodeposition coating material with a coat situated above it, theprocess of what is known as wet-on-wet application of electrodepositioncoating material and at least one further coat is known, for example,from the patent applications EP 0 817 684 A1, EP 0 639 660 A1, EP 0 595186 A1, EP 0 646 420 A1 or DE 41 26 476 A1. The coating materials whichare applied wet-on-wet may be in liquid (aqueous, conventional or powderslurry) or powder form. The coating materials may be pigmented andunpigmented and may be used to produce surfacers or functional coats(pigmented) or clearcoats (unpigmented), but especially to producesurfacers.

During the implementation of the wet-on-wet processes, the appliedelectrodeposition coating film is generally predried prior to theapplication of the next coating material. This is generally done underconditions in which water and solvents are largely evaporated from theelectrodeposition coating film. This procedure is environmentally andeconomically advantageous and, moreover, generally producesbetter-quality coatings.

Nevertheless, it is possible again and again to observe problems withthe surface appearance (i.e., appearance of the overall system includingclearcoat). These problems are manifested, for example, in the values ofa longwave/shortwave wavescan (light reflection) which gives a value forthe amount of scattered light. The flow of the coated material, as well,in many cases fails to meet requirements.

Attempts have been made to solve these problems, in a very wide varietyof ways.

For example, in the process according to the German Patent ApplicationDE 41 26 476 A1, the use of electrodeposition coating materials isrestricted to those which on curing have a baking loss of less than 10%.However, this imposes severe restrictions on the user in the selectionof suitable electrodeposition coating materials.

The process according to the European Patent Application EP 0 646 420 A1uses electrodeposition coating materials and powder coating materialswhose baking temperatures are harmonized with one another. Thus, theinterval of the minimum baking temperature of the second coat (powdercoat) should lie above the interval of the first coat (electrodepositioncoat), or the intervals should overlap such that the lower limit of theinterval of the minimum baking temperature of the second coat lies abovethe lower limit of the interval of the electrodeposition coat. In otherwords, the electrodeposition coating material has a baking temperaturewhich is lower than the baking temperature of the powder coatingmaterial. Despite this adaptation of the baking temperatures, problemsof appearance and of flow continue to occur. Moreover, extensive flakingmay occur on stone impact.

Accordingly, the attempts to solve the problems stated have essentiallyconcentrated on selecting only electrodeposition coating materialshaving a low volume shrinkage or on adapting to one another the bakingtemperatures of the electrodeposition coating film and the secondcoating film.

SUMMARY OF THE INVENTION

It is an object of the present invention to find a new process of thevariety mentioned at the outset for producing multicoat systems onelectrically conductive substrates which no longer has the disadvantagesof the prior art but which instead, in an environmentally andeconomically efficient manner, produces high-quality multicoat systemswhich have an improved surface appearance (appearance of the overallsystem including clearcoat) and better flow of the coating. The improvedappearance should be manifested significantly in particular in thevalues of a longwave/shortwave wavescan (light reflection) which gives avalue for the amount of scattered light. Moreover, the antistonechipproperties should be improved. A further aspect of the present inventionis the use of the multicoat systems in automobile coating and inindustrial coating.

This object is achieved by means of a process for producing a multicoatsystem on a substrate or the use of this multicoat system, in which

-   a) an electrodeposition coating film is deposited on the substrate,-   b) the electrodeposition coating film is predried by heating to a    predrying temperature of the electrodeposition coating material for    a predetermined period,-   c) a coat of a surfacer is applied to the electrodeposition coating    film, and-   d) the electrodeposition coating film and the coat of the surfacer    are baked together at elevated temperatures.

A feature of the process is that the predrying temperature lies at orabove, preferably from 0° C. to 35° C. and more preferably from 5° C. to25° C. above, the temperature T_(p) at which the loss factor tan δ ofthe unbaked (i.e., uncrosslinked) electrodeposition coating materialshows a maximum.

The recoverable energy component (elastic component) in the deformationof a viscoelastic material such as a polymer is determined by the sizeof the storage modulus E′, whereas the energy component consumed(dissipated) in this process is described by the size of the lossmodulus E″. The moduli E′ and E″ are dependent on the rate ofdeformation and the temperature. The loss factor tan δ is defined as thequotient formed from the loss modulus E″ and the storage modulus E′. tanδ may be determined with the aid of dynamic mechanical thermoanalysis(DMTA) and represents a measure of the relationship between the elasticand plastic properties of the electrodeposition coating film (Th. Frey,K. -H. Groβe-Brinkhause, U. Röckrath: Cure Monitoring of ThermosetCoatings, Progress in Organic Coatings 27 (1996) 59–66).

It has surprisingly been found that the achievement or exceedance of theabove-mentioned temperature T_(p) is critical to the success ofwet-on-wet application and that the evaporation of water or solvents isof little or no priority. Processes whose predrying operation is aimedonly at removing solvents, such as drying with predried air at reducedtemperatures, for example, therefore generally give results which aremuch poorer.

When the predrying temperature of the invention is kept to, an improvedsurface appearance may be observed (appearance of the overall systemincluding clearcoat). This is manifested, for example, in the values ofa longwave/shortwave wavescan (light reflection) which gives a value forthe amount of scattered light. The flow of the coating material is alsoimproved.

In addition, it is possible to observe an improvement in theantistonechip properties. In particular, the area of flaking is smallerand there is better adhesion to the substrate.

The reasons for the decisive influence of the predrying temperatureattained are unelucidated. It is possible that relaxation processes takeplace within the electrodeposition coating film (cf. Encyclopedia ofPolymer Science and Engineering, Vol. 5, John Wiley and Sons, pages299–329). The factor involved need not necessarily include glasstransitions, since with certain electrodeposition coating materials itwas possible in experiments to rule out explicitly such a glasstransition within the temperature range of the predrying.

In the process of the invention, it is possible in many cases toprescribe lower predrying temperatures than in the case of the priorart. In these cases, therefore, it is not necessary to cool down thesystem so greatly before applying the surfacer, so simplifying theprocess, saving energy, and leading to lower capital costs and operatingcosts.

It is a particular advantage of the process of the invention that theoptimum predrying temperature—irrespective of whether it iscomparatively high or comparatively low—may be determined in a simplemanner for a given electrodeposition coating material.

DETAILED DESCRIPTION OF THE INVENTION

The prescribed period for the implementation of predrying in step b) istypically from 1 to 60 minutes, preferably from 5 to 15 minutes. Afterpredrying, the substrate is preferably taken back to the ambienttemperature before the surfacer is applied. The time between theapplication of the electrodeposition coating material and theapplication of the surfacer is arbitrary.

In accordance with one development of the process of the invention, thecoat of a surfacer applied in step c) is predried for from about 1 to 30minutes, preferably for from 10 to 20 minutes, before the conjointbaking in step d). This predrying takes place at a temperature which isdependent on the surfacer material, so that the skilled worker isreadily able to determine the optimum temperature on the basis of his orher general knowledge in the art, possibly with the aid of rangefindingtests.

The thickness of the fully cured electrodeposition coating film ispreferably from 10 μm to 30 μm, with particular preference from 15 μm to20 μm. The thickness of the fully cured surfacer coat depends on thesurfacer material and is preferably from 10 μm to 60 μm.

Suitable baths for the electrodeposition coating operation are allcustomary anodic or cathodic electrodeposition coating baths.

These electrodeposition coating baths are aqueous coating materialshaving a solids content of in particular from 5 to 30% by weight.

The solids of the electrodeposition coating material comprise

-   (A) customary and known binders which carry functional groups (a1)    which are ionic or are convertible into ionic groups, and functional    groups (a2) capable of chemical crosslinking, these groups being    externally crosslinking and/or self-crosslinking, but especially    externally crosslinking;-   (B) if desired, crosslinking agents which carry complementary    functional groups (b1) which are able to enter into chemical    crosslinking reactions with the functional groups (a2), and which    are employed mandatorily when the binders (A) are externally    crosslinking; and-   (C) customary and known coatings additives.

Where the crosslinking agents (B) and/or their functional groups (b1)have already been incorporated into the binders (A), self-crosslinkingapplies.

Suitable complementary functional groups (a2) of the binders (A) arepreferably thio, amino, hydroxyl, carbamate, allophanate, carboxyland/or (meth)acrylate groups, but especially hydroxyl groups, andsuitable complementary functional groups (b1) are preferably anhydride,carboxyl, epoxy, blocked isocyanate, urethane, methylol, methylol ether,siloxane, amino, hydroxyl and/or beta-hydroxyalkylamide groups, butespecially blocked isocyanate groups.

Examples of suitable functional groups (a1), which are ionic or areconvertible into ionic groups, of the binders (A) are

-   (a11) functional groups which can be converted into cations by    neutralizing agents and/or quaternizing agents, and/or cationic    groups, or-   (a12) functional groups which can be converted into anions by    neutralizing agents, and/or anionic groups.

The binders (A) containing functional groups (a11) are used in cathodicelectrodeposition coating materials whereas the binders (A) containingfunctional groups (a12) are employed in anodic electrodeposition coatingmaterials.

Examples of suitable functional groups (a11) for use in accordance withthe invention that can be converted into cations by neutralizing agentsand/or quaternizing agents are primary, secondary or tertiary aminogroups, secondary sulfide groups or tertiary phosphine groups,especially tertiary amino groups or secondary sulfide groups.

Examples of suitable cationic groups (a11) for use in accordance withthe invention are primary, secondary, tertiary or quaternary ammoniumgroups, tertiary sulfonium groups or quaternary phosphonium groups,preferably quaternary ammonium groups or quaternary ammonium groups,tertiary sulfonium groups, but especially quaternary ammonium groups.

Examples of suitable functional groups (a12) for use in accordance withthe invention that may be converted into anions by neutralizing agentsare carboxylic, sulfonic or phosphonic acid groups, especiallycarboxylic acid groups.

Examples of suitable anionic groups (a12) for use in accordance with theinvention are carboxylate, sulfonate or phosphonate groups, especiallycarboxylate groups.

The selection of the groups (a11) or (a12) should be made so as to ruleout the possibility of any disruptive reactions with the functionalgroups (a2) which are able to react with the crosslinking agents (B).The skilled worker will therefore be able to make the selection in asimple way on the basis of his or her knowledge of the art.

Examples of suitable neutralizing agents for functional groups (a11)convertible into cations are organic and inorganic acids such assulfuric acid, hydrochloric acid, phosphoric acid, amidosulfonic acid,lactic acid, dimethylolpropionic acid, or citric acid, especially formicacid, acetic acid or lactic acid.

Examples of suitable neutralizing agents for functional groups (a12)convertible into anions are ammonia, ammonium salts, such as, forexample, ammonium carbonate or ammonium hydrogen carbonate and alsoamines, such as, for example, trimethylamine, triethylamine,tributylamine, dimethylaniline, diethylaniline, triphenylamine,dimethylethanolamine, diethylethanolamine, methyldiethanolamine,triethanolamine and the like.

In general, the amount of neutralizing agent is chosen so that from 1 to100 equivalents, preferably from 50 to 90 equivalents, of the functionalgroups (a11) or (a12) of the binder (b1) are neutralized.

Examples of suitable binders (A) for anodic electrodeposition coatingmaterials are known from the patent DE 28 24 418 A1. They comprise,preferably, polyesters, epoxy resin esters, poly(meth)acrylates, maleateoils or polybutadiene oils having a weight average molecular weight offrom 300 to 10 000 daltons and an acid number of from 35 to 300 mgKOH/g.

Examples of suitable binders (A) for cathodic electrodeposition coatingmaterials are known from the patents EP 0 082 291 A1, EP 0 234 395 A1,EP 0 227 975 A1, EP 0 178 531 A1, EP 0 333 327, EP 0 310 971 A1, EP 0456 270 A1, U.S. Pat. No. 3,922,253 A, EP 0 261 385 A1, EP 0 245 786 A1,EP 0 414 199 A1, EP 0 476 514 A1, EP 0 817 684 A1, EP 0 639 660 A1, EP 0595 186 A1, DE 41 26 476 A1, WO 98/33835, DE 33 00 570 A1, DE 37 38 220A1, DE 35 18 732 A1 or DE 196 18 379 A1.

These binders are preferably resins (A) containing primary, secondary,tertiary or quaternary amino or ammonium groups and/or tertiarysulfonium groups and having amine numbers of preferably between 20 and250 mg KOH/g and a weight average molecular weight of preferably from300 to 10 000 daltons. It is preferred to use amino (meth)acrylateresins, amino epoxy resins, amino epoxy resins having terminal doublebonds, amino epoxy resins having primary and/or secondary hydroxylgroups, amino polyurethane resins, amino-containing polybutadieneresins, or modified epoxy resin-carbon-dioxide-amine reaction products.

Particularly preferred resins used as binders (A) are modified epoxyresins in accordance with WO 98/33835, which are obtainable by reactingan epoxy resin with a mixture of monophenols and diphenols, reacting theresulting product with a polyamine to give an amino epoxy resin, andthen reacting the resulting amino epoxy resin in a further stage with anorganic amine to give the modified epoxy resin (cf. WO 98/33835, page19, line 1 to page 21, line 30).

In accordance with the invention, it is preferred to use cathodicelectrodeposition coating materials, especially cathodicelectrodeposition coating materials based on the above-described binders(A), and the corresponding electrodeposition coating baths.

The electrodeposition coating materials preferably comprise crosslinkingagents (B).

Examples of suitable crosslinking agents (B) whose use is preferred areblocked organic polyisocyanates, especially blocked so-called paintpolyisocyanates, containing blocked isocyanate groups attached toaliphatic, cycloaliphatic, araliphatic and/or aromatic moieties.

For their preparation, preference is given to using polyisocyanateshaving from 2 to 5 isocyanate groups per molecule and having viscositiesof from 100 to 10 000, preferably from 100 to 5000 and in particularfrom 100 to 2000 mPa·s (at 23° C.). Moreover, the polyisocyanates mayhave been hydrophilically or hydrophobically modified in a customary andknown manner.

Examples of suitable polyisocyanates are described, for example, in“Methoden der organischen Chemie”, Houben-Weyl, Volume 14/2, 4thedition, Georg Thieme Verlag, Stuttgart 1963, pages 61 to 70, and by W.Siefken, Liebigs Annalen der Chemie, Volume 562, pages 75 to 136.

Further examples of suitable polyisocyanates are isophorone diisocyanate(i.e. 5-isocyanato-1-isocyanatomethyl-1,3,3-trimethylcyclohexane),5-isocyanato-1-(2-isocyanatoeth-1-yl)-1,3,3-trimethylcyclohexane,5-isocyanato-1-(3-isocyanatoeth-1-yl)-1,3,3-trimethylcyclohexane,5-isocyanato-(4-isocyanatobut-1-yl)-1,3,3-trimethylcyclohexane,1-isocyanato-2-(3-isocyanatoprop-1-yl)cyclohexane,1-isocyanato-2-(3-isocyanatoeth-1-yl)cyclohexane,1-isocyanato-2-(4-isocyanatobut-1-yl)cyclohexane,1,2-diisocyanatocyclobutane, 1,3-diisocyanatocyclobutane,1,2-diisocyanatocyclopentane, 1,3-diisocyanatocyclopentane,1,2-diisocyanatocyclohexane, 1,3-diisocyanatocyclohexane,1,4-diisocyanatocyclohexane, dicyclohexylmethane 2,4′-diisocyanate,dicyclohexylmethane 4,4′-diisocyanate, liquid dicyclohexylmethane4,4′-diisocyanate with a trans/trans content of up to 30% by weight,preferably 25% by weight and in particular 20% by weight, obtainable byphosgenating isomer mixtures of bis(4-aminocyclohexyl)methane or byfractionally crystallizing commercially customarybis(4-isocyanatocyclohexyl)methane in accordance with the patents DE 4414 032 A1, GB 1220717 A1, DE 16 18 795 A1 or DE 17 93 785 A1;trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylenediisocyanate, hexamethylene diisocyanate, ethylethylene diisocyanate,trimethylhexane diisocyanate, heptamethylene diisocyanate ordiisocyanates derived from dimeric fatty acids, as marketed under thecommercial designation DDI 1410 by the company Henkel and described inthe patents WO 97/49745 and WO 97/49747, especially2-heptyl-3,4-bis(9-isocyanatononyl)-1-pentylcyclohexane, 1,2-, 1,4- or1,3-bis(isocyanatomethyl)cyclohexane, 1,2-, 1,4- or1,3-bis(2-isocyanatoeth-1-yl)cyclohexane,1,3-bis-(3-isocyanatoprop-1-yl)cyclohexane or 1,2-, 1,4- or1,3-bis(4-isocyanatobut-1-yl)cyclohexane, m-tetramethylxylylenediisocyanate (i.e. 1,3-bis(2-isocyanatoprop-2-yl)benzene or tolylenediisocyanate.

Examples of suitable polyisocyanate adducts are isocyanato-functionalpolyurethane prepolymers which are preparable by reacting polyols withan excess of polyisocyanates and are preferably of low viscosity. It isalso possible to use polyisocyanates containing isocyanurate, biuret,allophanate, iminooxadiazinedione, urethane, urea, carbodiimide and/oruretdione groups. Polyisocyanates containing urethane groups, forexample, are obtained by reacting some of the isocyanate groups withpolyols, such as trimethylolpropane and glycerol, for example.

Very particular preference is given to the use of mixtures ofpolyisocyanate adducts containing uretdione and/or isocyanurate and/orallophanate groups and based on hexamethylene diisocyanate, as areformed by catalytic oligomerization of hexamethylene diisocyanate usingappropriate catalysts. Moreover, the polyisocyanate constituent maycomprise any desired mixtures of the free polyisocyanates exemplified.

Examples of suitable blocking agents for preparing the blockedpolyisocyanates (B) are the blocking agents known from the U.S. Pat. No.4,444,954 A or U.S. Pat. No. 5,972,189 A, such as

-   i) phenols such as phenol, cresol, xylenol, nitrophenol,    chlorophenol, ethylphenol, t-butylphenol, hydroxybenzoic acid,    esters of this acid, or 2,5-di-tert-butyl-4-hydroxytoluene;-   ii) lactams, such as ε-caprolactam, δ-valerolactam, γ-butyrolactam    or β-propiolactam;-   iii) active methylenic compounds, such as diethyl malonate, dimethyl    malonate, ethyl or methyl acetoacetate, or acetylacetone;-   iv) alcohols such as methanol, ethanol, n-propanol, isopropanol,    n-butanol, isobutanol, t-butanol, n-amyl alcohol, t-amyl alcohol,    lauryl alcohol, ethylene glycol monomethyl ether, ethylene glycol    monoethyl ether, ethylene glycol monopropyl ester, ethylene glycol    monobutyl ether, diethylene glycol monomethyl ether, diethylene    glycol monoethyl ether, diethylene glycol monopropyl ether,    diethylene glycol monobutyl ether, propylene glycol monomethyl    ether, methoxymethanol, 2-(hydroxyethoxy)phenol,    2-(hydroxypropoxy)phenol, glycolic acid, glycolic esters, lactic    acid, lactic esters, methylolurea, methylolmelamine, diacetone    alcohol, ethylenechlorohydrin, ethylenebromohydrin,    1,3-dichloro-2-propanol, 1,4-cyclohexyldimethanol or    acetocyanohydrin;-   v) mercaptans such as butyl mercaptan, hexyl mercaptan, t-butyl    mercaptan, t-dodecyl mercaptan, 2-mercaptobenzothiazole, thiophenol,    methylthiophenol or ethylthiophenol;-   vi) acid amides such as acetoanilide, acetoanisidinamide,    acrylamide, methacrylamide, acetamide, stearamide or benzamide;-   vii) imides such as succinimide, phthalimide or maleimide;-   viii)amines such as diphenylamine, phenylnaphthylamine, xylidine,    N-phenylxylidine, carbazole, aniline, naphthylamine, butylamine,    dibutylamine or butylphenylamine;-   ix) imidazoles such as imidazole or 2-ethylimidazole;-   x) ureas such as urea, thiourea, ethyleneurea, ethylenethiourea or    1,3-diphenylurea;-   xi) carbamates such as phenyl N-phenylcarbamate or 2-oxazolidone;-   xii) imines such as ethyleneimine;-   xiii)oximes such as acetone oxime, formaldoxime, acetaldoxime,    acetoxime, methyl ethyl ketoxime, diisobutyl ketoxime, diacetyl    monoxime, benzophenone oxime or chlorohexanone oximes;-   xiv) salts of sulfurous acid such as sodium bisulfite or potassium    bisulfite;-   xv) hydroxamic esters such as benzyl methacrylohydroxamate (BMH) or    allyl methacrylohydroxamate; or-   xvi) substituted pyrazoles, imidazoles or triazoles; and also    mixtures of these blocking agents, especially dimethylpyrazole and    triazoles, malonic esters and acetoacetic esters, dimethylpyrazole    and succinimide, or butyl diglycol and trimethylolpropane.

Further examples of suitable crosslinking agents (B) are all knownaliphatic and/or cycloaliphatic and/or aromatic polyepoxides, based forexample on bisphenol A or bisphenol F. Examples of suitable polyepoxidesalso include the polyepoxides obtainable commercially under thedesignations Epikote® from Shell, Denacol® from Nagase Chemicals Ltd.,Japan, such as, for example, Denacol EX-411 (pentaerythritolpolyglycidyl ether), Denacol EX-321 (trimethylolpropane polyglycidylether), Denacol EX-512 (polyglycerol polyglycidyl ether), and DenacolEX-521 (polyglycerol polyglycidyl ether).

As crosslinking agents (B) it is also possible to usetris(alkoxycarbonylamino)triazines (TACT) of the general formula

Examples of suitable tris (alkoxycarbonylamino) triazines (B) aredescribed in the patents U.S. Pat. Nos. 4,939,213 A, U.S. Pat. No.5,084,541 A, and EP 0 624 577 A1. Use is made in particular of thetris(methoxy-, tris(butoxy- and/ortris(2-ethylhexoxycarbonylamino)triazines.

The methyl/butyl mixed esters, the butyl 2-ethylhexyl mixed esters, andthe butyl esters are of advantage. They have the advantage over thestraight methyl ester of better solubility in polymer melts, and alsohave less of a tendency to crystallize out.

Further examples of suitable crosslinking agents (B) are amino resins,examples being melamine resins, guanamine resins, benzoguanamine resinsor urea resins. Also suitable are the customary and known amino resinssome of whose methylol and/or methoxymethyl groups have beendefunctionalized by means of carbamate or allophanate groups.Crosslinking agents of this kind are described in the patents U.S. Pat.No. 4,710,542 A and EP 0 245 700 B1 and also in the article by B. Singhand coworkers, “Carbamylmethylated Melamines, Novel Crosslinkers for theCoatings Industry” in Advanced Organic Coatings Science and TechnologySeries, 1991, Volume 13, pages 193 to 207.

Further examples of suitable crosslinking agents (B) arebeta-hydroxyalkylamides such asN,N,N′,N′-tetrakis(2-hydroxyethyl)adipamide orN,N,N′,N′-tetrakis-(2-hydroxypropyl)adipamide.

Further examples of suitable crosslinking agents (B) are compoundscontaining on average at least two groups capable oftransesterification, examples being reaction products of malonicdiesters and polyisocyanates or of esters and partial esters ofpolyhydric alcohols of malonic acid with monoisocyanates, as describedin the European patent EP 0 596 460 A1.

The amount of the crosslinking agents (B) in the electrodepositioncoating material may vary widely and is guided in particular, firstly,by the functionality of the crosslinking agents (B) and, secondly, bythe number of crosslinking functional groups (a) which are present inthe binder (A), and also by the target crosslinking density. The skilledworker is therefore able to determine the amount of the crosslinkingagents (B) on the basis of his or her general knowledge in the art,possibly with the aid of simple rangefinding experiments.Advantageously, the crosslinking agent (B) is present in theelectrodeposition coating material in an amount of from 5 to 60, withparticular preference from 10 to 50, and in particular from 15 to 45% byweight, based in each case on the solids content of the coating materialof the invention. It is further advisable here, to choose the amounts ofcrosslinking agent (B) and binder (A) such that in the electrodepositioncoating material the ratio of functional groups (b1) in the crosslinkingagent (B) to functional groups (a2) in the binder (A) is from 2:1 to1:2, preferably from 1.5:1 to 1:1.5, with particular preference from1.2:1 to 1:1.2, and in particular from 1.1:1 to 1:1.1.

The electrodeposition coating material may comprise customary coatingadditives (C) in effective amounts.

Examples of suitable additives (C) for pigmented electrodepositioncoating materials are

-   -   organic and/or inorganic pigments, anticorrosion pigments and/or        fillers such as calcium sulfate, barium sulfate, silicates such        as talc or kaolin, silicas, oxides such as aluminum hydroxide or        magnesium hydroxide, nanoparticles, organic fillers such as        textile fibers, cellulose fibers, polyethylene fibers, titanium        dioxide, carbon black, iron oxide, zinc phosphate or lead        silicate; these additives may also be incorporated into the        electrodeposition coating materials of the invention by way of        pigment pastes.

These additives (C) are of course not present in the unpigmentedelectrodeposition coating materials.

Examples of additives (C) which may be present both in pigmented and inunpigmented electrodeposition coating materials are

-   -   free-radical scavengers;    -   organic corrosion inhibitors;    -   crosslinking catalysts such as organic and inorganic salts and        complexes of tin, lead, antimony, bismuth, iron or manganese,        preferably organic salts and complexes of bismuth and of tin,        especially bismuth lactate, citrate, ethylhexanoate or        dimethylol-propionate, dibutyltin oxide or dibutyltin dilaurate;    -   slip additives;    -   polymerization inhibitors;    -   defoamers;    -   emulsifiers, especially nonionic emulsifiers such as alkoxylated        alkanols and polyols, phenols and alkylphenols or anionic        emulsifiers such as alkali metal salts or ammonium salts of        alkanecarboxylic acids, alkanesulfonic acids, and sulfo acids of        alkoxylated alkanols and polyols, phenols and alkylphenols;    -   wetting agents such as siloxanes, fluorine compounds, carboxylic        monoesters, phosphoric esters, polyacrylic acids and their        copolymers, or polyurethanes;    -   adhesion promoters;    -   leveling agents;    -   film-forming auxiliaries such as cellulose derivatives;    -   flame retardants;    -   organic solvents;    -   low molecular mass, oligomeric and high molecular mass reactive        diluents which are able to participate in the thermal        crosslinking, especially polyols such as        tricyclodecanedimethanol, dendrimeric polyols, hyperbranched        polyesters, polyols based on metathesis oligomers or on branched        alkanes having more than eight carbon atoms in the molecule;    -   anticrater agents;

Further examples of suitable coatings additives are described in thetextbook “Lackadditive” [Additives for coatings] by Johan Bieleman,Wiley-VCH, Weinheim, N.Y., 1998.

The above-described crosslinking agents (B) and/or the above-describedadditives (C) may also be present in the surfacers described below.

In accordance with the invention, lead-free cathodic electrodepositioncoating materials afford particular advantages and are therefore usedwith preference.

For the preparation of the electrodeposition coating film, veryparticular preference is given to the use of an electrodepositioncoating material which comprises

-   (A) at least one modified epoxy resin as binder, preparable by    reacting epoxy resin with a mixture of monophenols and diphenols,    reacting the resulting product with a polyamine to give an amino    epoxy resin, and then reacting the resulting amino epoxy resin in a    further stage with a further polymamine to give the modified epoxy    resin; and-   (B) at least one blocked polyisocyanate as crosslinking agent. The    predrying temperature in step b) in that case is from 70° C. to 120°    C., preferably from 80° C. to 100° C.

It is a very important advantage of the process of the invention that itis possible to use even those of the above-described electrodepositioncoating materials which suffer a baking loss of more than 10% on curing,without the occurrence of the problems mentioned at the outset.

Examples of suitable surfacers or antistonechip primers are known fromthe patents U.S. Pat. No. 4,537,926 A1, EP 0 529 335 A1, EP 0 595 186A1, EP 0 639 660 A1, DE 44 38 504 A1, DE 43 37 961 A1, WO 89/10387, U.S.Pat. Nos. 4,450,200 A1, 4,614,683 A1, WO 94/26827 or EP 0 788 523 B1.The surfacers in this case may be present as conventional—i.e.,solventborne—or as aqueous coating materials. It is also possible to usepowder coating materials or powder slurry coating materials.

Preference is given to the use of aqueous surfacers.

It is preferred to use aqueous surfacers comprising as binder a waterdilutable polyurethane resin. Particular preference is given to aqueoussurfacers based on water dilutable polyurethane resins obtainable byreacting with one another polyester polyols and/or polyether polyols,polyisocyanates, compounds containing at least one isocyanate-reactivegroup and at least one (potentially) anionic group in the molecule, andalso, if desired, compounds containing hydroxyl and/or amino groups. Toprepare the surfacer, the polyurethane resin is neutralized at leastpartly and dispersed in water. The dispersion is then made up withpigments and crosslinking agents (cf., for example, the European patentEP 0 788 523 B1, page 5, lines 1 to 29).

The function of the surfacer or of the coating produced from it is toeven out disruptive unevennesses (in the micrometer range) on thesurface of a substrate, so that the surface of the substrate need not besubjected to a leveling pretreatment prior to the application of acoating. This is also done using the comparatively high coat thicknessof the surfacer. It additionally serves to absorb and dissipatemechanical energy, as occurs on stone impact.

The multicoat system produced by the process of the invention may beused per se (2-coat system) for the abovementioned end uses. However, itmay also be overcoated with a clearcoat or solid-color topcoat, giving a3-coat system which offers an economical alternative to comparativelyexpensive coatings. For demanding applications where a particularly goodappearance is critical, the multicoat system produced by the process ofthe invention may further be coated with a color and/or effectbasecoat/clearcoat system, preferably by the wet-on-wet technique(4-coat system).

These processes for producing multicoat systems are being usedincreasingly in industrial coating for articles of all types inindustrial and private use. Examples of such articles are radiators,wheelrims or hubcaps. However, they may also be used to coat automobilebodies.

The invention additionally relates to a method of determining thepredrying temperature of the electrodeposition coating material in aprocess for producing a multicoat system of the type specified at theoutset. In the method, a determination is made of the temperature T_(p)at which a viscoelastic property of the as yet unbaked electrodepositioncoating material exhibits an extreme value, and the predryingtemperature is chosen to be the same as or above, preferably from 0° C.to 35° C. and more preferably from 5° C. to 25° C. above, thistemperature T_(p).

The finding underlying this method is that the primary factor in thepredrying of an electrodeposition coating material in the course of thewet-on-wet application of an electrodeposition coating material and asurfacer is not the evaporation of the solvents, but instead that it isimportant to exceed the temperature at which internal changes of theelectrodeposition coating material take place which manifest themselvesin an extreme value of a viscoelastic property.

The viscoelastic property of the electrodeposition coating material thatis considered in this case is the loss factor tan δ. An improvement inthe coating result at predrying temperatures above the maximum of theloss factor tan δ has been demonstrated in numerous experiments.

The DMTA is a widely known measurement method for determining theviscoelastic properties of coatings and is described, for example, inMurayama, T., Dynamic Mechanical Analysis of Polymeric Materials,Elsevier, N.Y., 1978, pages 299 to 329 and Loren W. Hill, Journal ofCoatings Technology, Vol. 64, No. 808, May 1992, pages 31 to 33. Theprocess conditions during the measurement of tan δ by means of DMTA aredescribed in detail by Th. Frey, K. -H. Groβe-Brinkhaus, U. Röckrath, inCure Monitoring of Thermoset Coatings, Progress In Organic Coatings 27(1996) 59–66, or in DE 44 09 715 A1.

As well as in DMTA, such a signal may also be found in the course ofviscosity measurements of wet cathodic electrodeposition coating filmsand the corresponding evaluation in accordance with tan δ=G″/G′ (cf. cf.T. Dirking, K. -H. Groβe-Brinkhaus, Rheologische Charakterisierung vonElektrotauchlacken während des Einbrennvorgangs: Korrelation vonViskositätswerten mit Kantenschutzergebnissen [Rheologicalcharacterization of electrodeposition coating materials during thebaking procedure: correlation of viscosity values with edge protectionresults], Fatipec XXIII, 1996, Brussels, Belgium, pages B-260 to B-271).Since these measurements are generally taken after predrying at from100° C. to 110° C., it is evident here too that this is a characteristicof the cathodic electrodeposition coating system.

The particular advantages of the process of the invention are not,however, restricted to the combination of electrodeposition coating andsurfacer coating but instead extend to the coatings lying above them aswell. Accordingly, the clearcoats, solid-color topcoats or color and/oreffect basecoat/clearcoat systems produced on top of them have animproved surface appearance (appearance of the overall system includingclearcoat). This is manifested, for example, in the values of alongwave/shortwave wavescan (light reflection) which gives a value forthe amount of scattered light. The flow of the coating material is alsoimproved.

Moreover, an improvement may be observed in the antistonechipproperties. In particular, the flaking area is smaller and there isbetter adhesion to the substrate.

Inventive Examples

1. Preparation of a Crosslinking Agent (C1) for an ElectrodepositionCoating Material

A reactor is charged under nitrogen with 10 462 parts of isomers andoligomers of higher functionality based on 4,4′-diphenylmethanediisocyanate, having an NCO equivalent weight of 135 g/eq (Lupranat®M20S from BASF AG; NCO functionality approx. 2.7; 2,2′- and2,4′-diphenylmethane diisocyanate content less than 5%). 20 parts ofdibutyltin dilaurate are added and 9626 parts of butyl diglycol areadded dropwise at a rate such that the product temperature remains below60° C. Following the addition, the temperature is held at 60° C. for afurther 60 minutes and an NCO equivalent weight of 1120 g/eq is measured(based on solid fractions). After the product has been diluted in 7737parts of methyl isobutyl ketone, and 24 parts of dibutyltin dilauratehave been added, 867 parts of melted trimethylolpropane are added at arate such that the product temperature does not exceed 100° C. Followingthe addition, reaction is continued for 60 minutes. The mixture iscooled to 65° C. and simultaneously diluted with 963 parts of n-butanoland 300 parts of methyl isobutyl ketone. The solids content is 70.1% (1h at 130° C.).

2. Preparation of a Precursor (AC1) for an Electrodeposition CoatingBinder

The water of reaction is removed at from 110° C. to 140° C. from a 70%strength solution of diethylenetriamine in methyl isobutyl ketone. Thesolution is subsequently diluted with methyl isobutyl ketone until ithas an amine equivalent weight of 131 g/eq.

3. Preparation of an Aqueous Electrodeposition Coating Binder Dispersion(D1)

In a reactor fitted with a stirrer, reflux condenser, internalthermometer and inert gas inlet, 6150 parts of epoxy resin based onbisphenol A, having an epoxy equivalent weight (EEW) of 188, are heatedto 125° C. under nitrogen together with 1400 parts of bisphenol A, 355parts of dodecylphenol, 470 parts of p-cresol and 441 parts of xylene,and the mixture is stirred for 10 minutes. It is subsequently heated to130° C. and 23 parts of N,N-dilmethylbenzylamine are added. The reactionmixture is held at this temperature until the EEW has reached a value of880 g/eq.

Then a mixture of 7097 parts of the crosslinking agent C1 and 90 partsof the additive K 2000 (polyether from Byk Chemie, Germany) is added andthe mixture is held at 100° C. Half an hour later, 211 parts of butylglycol and 1210 parts of isobutanol are added.

Immediately thereafter, a mixture of 467 parts of the precursor AC1(from step 2) and 520 parts of methylethanolamine is introduced into thereactor and the mixture is conditioned to 100° C. After a furtherhalf-hour, the temperature is raised to 105° C. and 159 parts ofN,N-dimethylaminopropylamine are added.

75 minutes after the addition of the amine, 903 parts of Plastilit® 3060(propylene glycol compound from BASF AG) are added and the mixture isdiluted with 522 parts of propylene glycol phenyl ether (mixture of1-phenoxy-2-propanol and 2-phenoxy-1-propanol, from BASF AG) and at thesame time cooled rapidly to 95° C.

After 10 minutes, 14 821 parts of the reaction mixture are transferredto a dispersing vessel. There, 474 parts of lactic acid (88% strength inwater), dissolved in 7061 parts of deionized water, are added withstirring. The mixture is subsequently homogenized for 20 minutes beforebeing diluted further with an additional 12 600 parts of deionized waterin small portions.

The volatile solvents are removed by distillation under reduced pressureand then replaced by an equal volume of deionized water.

The dispersion D1 possesses the following characteristics:

Solids content: 33.8% (1 h at 130° C.) 29.9% (0.5 h at 180° C.) Basecontent: 0.71 milliequivalents/g solids (130° C.) Acid content: 0.36milliequivalents/g solids (130° C.) pH: 6.3 Particle size: 116 nm (massaverage from photon correlation spectroscopy)4. Preparation of the Aqueous Electrodeposition Binder Dispersion (D2)

The binder dispersion D2 is prepared in exactly the same way as thebinder dispersion D1 except that, immediately after the dilution withpropylene glycol phenyl ether, 378 parts of K-KAT 348 (bismuth2-ethylhexanoate from King Industries, USA) are admixed to the organicstage with stirring. After cooling, 14 821 parts of the reaction mixtureare dispersed in exactly the same way as dispersion D1.

The dispersion D2 possesses the following characteristics:

Solids content: 33.9% (1 h at 130° C.) 30.1% (0.5 h at 180° C.) Basecontent: 0.74 milliequivalents/g solids (130° C.) Acid content: 0.48milliequivalents/g solids (130° C.) pH: 5.9 Particle size: 189 nm (massaverage from photon correlation spectroscopy)5. Preparation of a Crosslinking Agent (C2) for an ElectrodepositionCoating Material (in Analogy to WO 98/33835)

A reactor is charged under nitrogen with 1084 g parts of isomers andoligomers of higher functionality based on 4,4′-diphenylmethanediisocyanate, having an NCO equivalent weight of 135 g/eq (Lupranat®M20S from BASF AG; NCO functionality approx. 2.7; 2,2′- and2,4′-diphenylmethane diisocyanate content less than 5%). 2 g ofdibutyltin dilaurate are added and 1314 g of butyl diglycol are addeddropwise at a rate such that the product temperature remains below 70°C. It may be necessary to carry out cooling. After the end of addition,the temperature is held at 70° C. for a further 120 minutes.

The solids content is >97% (1 h at 130° C.).

6. Preparation of the Aqueous Electrodeposition Coating BinderDispersion (D3) (in Analogy to WO 98/33835, Example 2.3)

In a reactor fitted with a stirrer, reflux condenser, internalthermometer and inert gas inlet, 1128 parts of epoxy resin based onbisphenol A, having an epoxy equivalent weight (EEW) of 188, 94 parts byweight of phenol and 228 parts of bisphenol A are heated to 127° C.under nitrogen. With stirring, 1.5 parts of triphenylphosphine areadded, whereupon an exothermic reaction occurs and the temperature risesto 160° C. The mixture is allowed to cool again to 130° C. and theepoxide content is monitored. The EEW of 532 indicates that >98 of thephenolic OH groups have reacted. The subsequent addition of 157 parts ofPluriol P 600 (polypropylene glycol MW 600, BASF) is accompanied bycooling. When the mixture reaches 95° C., 115.5 parts of diethanolamineare added, whereupon an exothermic reaction occurs and the temperaturerises to 115° C. After a further 40 minutes at 115° C., 61.2 parts ofN,N-dimethylaminopropylamine are added. Following a short exothermicperiod (T_(max) 140° C.), the mixture is left to react further for 2hours at 130° C. until the viscosity remains constant. Then 97.6 partsof butyl glycol and 812 parts of the crosslinking agent (C2) are added,with simultaneous cooling, and the mixture is discharged at 105° C.

2400 parts of the still-hot mixture are dispersed immediately withintensive stirring in a mixture of 2173 parts of fully deionized waterand 49.3 parts of glacial acetic acid. 53 parts of K-KAT 348 (bismuth2-ethylhexanoate from King Industries, USA) are added to this mixture,and the resulting mixture is homogenized briefly and then diluted with afurther 752 parts of fully deionized water, after which it is filteredthrough plate filter K 900 (from Seitz). The dispersion D3 has thefollowing characteristics:

Solids content: 45% (1 h at 130° C.) 40.1% (0.5 h at 180° C.) Basecontent: 0.82 milliequivalents/g solids (130° C.) Acid content: 0.42milliequivalents/g solids (130° C.) pH: 6.1 Particle size: 129 nm (massaverage from photon correlation spectroscopy)7. Preparation of an Epoxy-amine Adduct Solution that is Used (E1)

In accordance with Example 1.3 of EP 0 505 445 B1, an organic-aqueoussolution of an epoxy-amine adduct is prepared by reacting, in a firststage, 2598 parts of bisphenol A diglycidyl ether (epoxy equivalentweight (EEW): 188 g/eq), 787 parts of bisphenol A, 603 nparts ofdodecylphenol and 206 parts of butyl glycol in the presence of 4 partsof triphenylphosphine at 130° C. to an EEW of 865 g/eq. Cooling isaccompanied by dilution with 849 parts of butyl glycol and 1534 parts ofD.E.R.®732 (polypropylene glycol glycidyl ether from DOW Chemical), andreaction is continued at 90° C. with 266 parts of2,2′-aminoethoxyethanol and 212 parts of N,N-dimethylaminopropylamine.After 2 hours, the viscosity of the resin solution is constant (5.3dPa·s; 40% strength in Solvenon® PM (methoxypropanol from BASF AG); coneand plate viscometer at 23° C.). The product is diluted with 1512 partsof butyl glycol and the base groups are partly neutralized with 201parts of glacial acetic acid; the product is diluted further with 1228parts of deionized water and discharged.

This gives a 60% strength aqueous-organic resin solution whose 10%dilution has a pH of 6.0.

8. Preparation of a Pigment Paste

First of all, a premix is formed from 277 parts of water and 250 partsof the epoxy-amine adduct described in step 5. Then 5 parts of carbonblack, 60 parts of Extender ASP 200, 351 parts of titanium dioxideTI-PURE® R 900 (from DuPont) and 54 parts of dibutyltin oxide (Fascat4203 from Elf-Atochem) are added and mixed for 30 minutes under ahigh-speed dissolver stirring mechanism. The mixture is subsequentlydispersed in a stirred laboratory mill for from 1 to 1.5 h to a Hegmanfineness of 12 μm and is adjusted if necessary to the desired processingviscosity using further water. Solids content: 60% (0.5 h, 180° C.)

9. Preparation of the Electrodeposition Coating Materials

The electrodeposition coating binder dispersions D1–D3 and, ifappropriate, the pigment paste (step 8) were used to prepare thefollowing electrodeposition coating materials:

ETL1 ETL2 ETL3 Dispersion D1 2771 — — Dispersion D2 — 2492 — DispersionD3 — — 1871 Pigment paste  313 DI water 1916 2508 3129

The electrodeposition coating materials obtained in this way have asolids content of approximately 20% in the case of the pigmented systemETL1 and 15% in the case of the clearcoats ETL2–3.

The application conditions (deposition voltage, deposition temperature)were chosen so that, following bath aging of a minimum of 24 h andbaking (15 minutes at a panel temperature of 180° C.), smooth films witha thickness of approximately 20 μm were obtained on steel panels (e.g.,Bo 26 W 42 OC) which had not been given a passivating rinse.

The process conditions for the determination of T_(p) of theelectrodeposition coating materials by means of DMTA were as follows(cf. Th. Frey, K. -H. Groβe-Brinkhaus, U. Röckrath, Cure Monitoring ofThermoset Coatings, Progress in Organic Coatings 27 (1996) 59–66, or inDE 44 09 715 A1):

1. Preparation: deposition of electrodeposition coating material on acarbon fiber mesh (Sigratex from Sigri) 2. Instrument: DMA MK IV (fromRheometric Scientific) 3. Conditions: tensile mode, amplitude 0.2%,frequency 1 Hz 4. Temperature ramp: 1° C./min from room temperature to200° C.

In the DMTA, a sudden sharp change in the storage modulus E′ and theloss factor tan δ occurred at about 145° C., and can be attributed tothe beginning of the crosslinking reaction. It was also possible to seethat below this crosslinking temperature the loss factor tan δ shows amaximum (peak) at a temperature T_(p) of about 90° C. As was found byfurther investigations, this maximum was not attributable to a glasstransition in the cathodic electrodeposition coating material underconsideration.

During a repeat scan of the temperature range from 20° C. to 100° C.with a ten-minute holding time at 100° C. it was found that, even afterseveral runthroughs a peak at about 80° C. in the loss factor tan δoccurred in each case. Only a slight shift in the signal toward highertemperatures during the temperature cycles was observed, which pointedto evaporation or drying. The peak itself, however, was retained, sothat the basis for this signal could not lie in drying phenomena.

For the inventive experiments, the panels were not baked but insteadonly predried in a forced air oven for 10 minutes at 80° C. or at 100°C. The temperatures were chosen because the electrodeposition coatingfilms deposited, as described above, had a maximum of a loss factor tanδ at T_(p)>80° C.

10. Preparation of Water Dilutable Polyurethane Resins

The preparation is as in Example 1.1 of EP 0 788 523 B1.

11. Preparation of Aqueous Surfacers

The preparation is as in Example 2.a of EP 0 788 523 B1.

The aqueous coating material (step 11) is applied with a dry filmthickness of 19 μm to the predried cathodic electrodeposition coatingsand is itself predried at 70° C. Subsequently, cathodicelectrodeposition coating and the aqueous coating material are bakedtogether for 15 minutes at a panel temperature of 180° C.

For further tests (especially stonechip testing), the panels areovercoated with a commercially customary white basecoat material havinga dry film thickness of 18 μm and with a commercially customarytwo-component clearcoat material having a film thickness of 35–40 μm.These films are baked at 130° C. for 30 minutes.

In order to determine the overall appearance, a black basecoat materialwith a film thickness of 14 μm is used instead of the white basecoatmaterial.

The test results of the individual coating systems, as a function of thedrying conditions in particular, are collated in the tables below.

TABLE 1 Technological testing Test system: Cathodic electrodepositioncoating wet-on- wet with aqueous surfacer Application method: A.Predrying of cathodic electrodeposition coating at  80° C., B. Predryingof cathodic electrodeposition coating at 100° C., then conjoint bakingfor 15′ at 180° C. (panel temp.) and white topcoat system. ETL1 ETL2ETL3 T_(p): 89° C. ETL1 T_(p): 86° C. ETL2 T_(p): 82° C. ETL3Application A B A B A B VDA stone 2 1–2 1–2 1–2 2 1–2 chipping^(a)) MBball 7 6 9 5 40 5.5 bombardment flaking^(b)) MB ball 3 1 4 2 5 1bombardment degree of rusting^(b)) ^(a))VDA stonechipping: Multiplestonechip test in accordance with VDA [German automakers association]test sheet: best score = 0, worst score = 5 ^(b))MB ball bombardmenttest: Single impact testing in accordance with DaimlerChryslerspecification LPV 5200.40701 Flaking: instances of flaking, in mm²Degree of rusting: visual evaluation of the damage at the area offlaking; best score = 0, worst score = 5

The results of Table 1 underscore the significant improvement whichoccurs in the stonechip resistance when the electrodeposition coatingfilms are predried above the temperature T_(p).

TABLE 2 Effect of application on the surface appearance Test system:Cathodic electrodeposition coating wet-on- wet with aqueous coatingmaterial Application method: A. Predrying of cathodic electrodepositioncoating at  80° C., B. Predrying of cathodic electrodeposition coatingat 100° C., then conjoint baking for 15′ at 180° C. (panel temp.) andblack topcoat system. ETL1 ETL1 ETL2 ETL2 ETL3 ETL3 Application A B A BA B Wavescan Lw^(c)) 5 4 7.4 3.2 17 3.0 Wavescan Sw^(c)) 42 28.1 35 22.260 28.7 ^(c))Wavescan: Measurement of the waviness of the paintedsurface Instrument: “wave-scan plus” from Byk-Gardner Longwavecharacteristics (Lw, longwave) = structures > 0.6 mm Shortwavecharacteristics (Sw, shortwave) = structures < 0.6 mm Scale for Lw, Sw:0 to 99, with 0 denoting the best result

The results of Table 2 underscore the significant improvement whichoccurs in the appearance when the electrodeposition coating films arepredried above the temperature T_(p).

1. A process for producing a multicoat system on a substrate comprisinga) depositing an electrodeposition coating film on the substrate, b)predrying the electrodeposition coating film by heating to a predryingtemperature for a predetermined period, c) applying a coat of a surfacerto the electrodeposition coating film, and d) baking theelectrodeposition coating film and the coat of the surfacer together atelevated temperatures, wherein the predrying temperature in step b) isfrom 0° C. to 35° C. above a temperature (T_(p)) at which a loss factortan δ, which is the quotient formed from a loss module E″ and a storagemodulus E′, of the unbaked electrodeposition coating film shows amaximum.
 2. The process of claim 1, wherein the electrodepositioncoating film is prepared using an electrodeposition coating materialcomprising (A) a binder comprising at least one modified epoxy resin,wherein the modified epoxy resin comprises a reaction product of anamino epoxy resin and an organic amine, wherein the amino epoxy resincomprises a reaction product of a polyamine and an epoxy resin reactionproduct, wherein the epoxy resin reaction product comprises a reactionproduct of an epoxy resin and a mixture of monophenols and diphenols;and (B) a crosslinking agent comprising at least one blockedpolyisocyanate.
 3. The process of claim 2, wherein the predryingtemperature in step b) is from 70° C. to 120° C.
 4. The process of claim1, wherein the period of predrying in step b) is from 1 to 60 minutes.5. The process of claim 1, wherein the thickness of the bakedelectrodeposition coating film is from 10 μm to 30 μm.
 6. The process ofclaim 1, wherein the thickness of the coat of the surfacer after bakingis from 10 μm to 60 μm.
 7. The process of claim 1, wherein the substrateis cooled to ambient temperature before application of the coat of thesurfacer.
 8. The process of claim 1, wherein the electrodepositioncoating film is a cathodically depositable coating film.
 9. The processof claim 1, wherein the surfacer is an aqueous coating material.
 10. Theprocess of claim 9, wherein the surfacer comprises a binder comprising awater-soluble polyurethane resin.
 11. The process of claim 1, whereinone of i) the coat of the surfacer forms the topmost coat of themulticoat system (2-coat system), ii) the coat of the surfacer isovercoated with a clearcoat or solid-color topcoat (3-coat system), andiii) the coat of the surfacer is overcoated with a color and/or effectbasecoat/clearcoat system (4-coat system).
 12. The process of claim 1,wherein the multicoat system is one of an automobile coating and anindustrial coating.
 13. The process of claim 12, wherein the industrialcoating is one of a radiator coating, a wheel rim coating, and a hubcapcoating.
 14. A method of determining a predrying temperature of anelectrodeposition coating material in a process for producing amulticoat system comprising the steps of a) depositing anelectrodeposition coating film on a substrate, b) predrying theelectrodeposition coating film by heating at a predrying temperature fora prescribed period, c) applying a coat of a surfacer to theelectrodeposition coating film, and d) conjointly baking theelectrodeposition coating film and the coat of the surfacer at elevatedtemperatures, wherein a temperature (T_(p)) is determined at which aloss factor tan δ of the electrodeposition coating film, which is aquotient formed from a loss modulus E″ and a storage modulus E′, in theas yet unbaked state has an extreme value, and in that the predryingtemperature is chosen to be equal to or from 0° C. to 35° C. above thetemperature (T_(p)).
 15. The method of claim 14, wherein the predryingtemperature is chosen to be 5° C. to 25° C. above the temperature(T_(p)) at which the loss factor tan δ of the electrodeposition coatingfilm in the as yet unbaked state has an extreme value.
 16. The method ofclaim 14, wherein the loss factor tan δ is determined with the aid ofdynamic mechanical thermoanalysis (DMTA).